1. Field of the Invention
The present invention relates to control of a persistent image in a display device which uses, for example, an organic electroluminescence element as a display element in each pixel.
2. Description of the Related Art
Display devices which use an organic electroluminescence (EL) element, which is a current-driven light emitting element, as a display element in each pixel are known and, in particular, active matrix display devices in which a transistor (thin film transistor or “TFT”) is provided in each pixel for individually driving, for each pixel, the organic EL element provided in each pixel are currently under development.
In an active matrix display device, a gate line GL is provided along a horizontal scan direction (row direction), a data line DL and a power supply line PL are provided along a vertical scan direction (column direction), and pixels are defined by these lines. As the equivalent circuit of each pixel, a circuit shown in FIG. 1 is known in which each pixel comprises a selection transistor Ts which is an n-channel TFT, a storage capacitor Cs, a p-channel element driving transistor Td, and an organic EL element EL. The selection transistor Ts has a drain connected to a data line DL which supplies a data voltage to pixels positioned along the vertical scan direction, a gate connected to a gate line GL for selecting pixels positioned along the horizontal scan direction, and a source connected to a gate of the element driving transistor Td.
The element driving transistor Td is a p-channel TFT and has a source connected to the power supply line PL and a drain connected to an anode of the organic EL element EL. A cathode of the organic EL element EL is formed common to the pixels and is connected to a cathode power supply CV. One electrode of a storage capacitor Cs is connected between the gate of the element driving transistor Td and the source of the selection transistor Ts. The other electrode of the storage capacitor Cs is connected to a power supply of a constant voltage such as, for example, ground.
In this circuit, when the gate line GL is set to the H level, the selection transistor Ts is switched on, a data voltage on the data line DL is supplied via the selection transistor Ts to the gate of the element driving transistor Td, and a voltage corresponding to the data voltage is stored in the storage capacitor Cs. In this manner, the element driving transistor Td allows a drive current corresponding to the gate voltage (the voltage stored in the storage capacitor Cs) of the element driving transistor Td to flow through the element transistor Td, and even when the gate line GL is set to an L level, the element driving transistor Td supplies the drive current from the power supply line PL connected to a drive power supply PVDD to the organic EL element EL according to the voltage stored in the storage capacitor Cs, and thus as a result the organic EL element EL emits light at an intensity corresponding to the drive current.
Japanese Patent Laid-Open Publication Nos. Hei 11-24604 and 2003-150127 disclose art related to the present invention.
The above-described organic EL element has superior responsiveness with respect to supply and termination of supply of current, and fundamentally, the persistent image does not tend to occur. However, in the display device which uses a pixel circuit as described above, there is a problem in that persistent images occur and display quality is degraded. This is considered to be due to hysteresis of the p-channel element driving transistor. More specifically, the element driving transistor supplies a drive current from the power supply PVDD for approximately one frame period according to the data voltage stored in the storage capacitor and supplied to the gate of the element driving transistor and then, after the next data voltage is written to the storage capacitor Cs, supplies a drive current in the next frame period according to a new data voltage. Because the element driving transistor Td supplies the same current throughout one frame period in this manner, this state is retained and the influence of the data voltage written previously remains even after the next data voltage is supplied. This phenomenon becomes even more significant when the data voltage is at an intermediate level and becomes particularly problematic when an animated image having a large variation in data voltage is to be displayed.
The details of the cause of the occurrence of the persistent image are not fully understood at this point, but some conjecture has been made, such as that the carriers (holes) flowing in the channel of the element driving transistor are trapped in the gate insulating film and the threshold voltage of the element driving transistor is changed by the trapped carriers.